Impact and acceleration testing



Feb. 28; 1950 G. R. s EARs E11-AL 2,498,844

IMPACT AND ACCELEATION TESTING Filed Aug 16, 1947 Patented Feb. 28,- 1950 IMPACT AND AccELERATioN TESTING George R. Sears, Robert D. Rae, and Johannes A. Van den Akker, Appleton, Wis., assignors to The Institute of Paper Chemistry, Appleton, Wis., a corporation of Wisconsin Application August 16, 1947, serial No. 769,016

4 Claims.

The present invention relates to testing apparatus and, in particular, to improved methods and apparatus for determining impact and acceleration. The method and apparatus of the invention may be used for calibrating and testing acceleration meters, impact recorders, fG meters, and the like, or it may be used to subject containers and packing materials to known impact accelerations in a manner similar to that encountered during shipping.

Heretofore, tests as described above have been generally carried out by equipment which was either inaccurate or was extremely expensive. Impact tests on shipping containers, packing Ina- 4terials, and methods of interior bracing of cartons, usually have been made on apparatus which includes an inclined plane, down which rolls a dollyY that carries the object under test into impact with a massive bulkwark. With such equipment, the results obtained are not reproducible because of the appreciabley friction and side-sway of the dolly and its attached test specimen. Furthermore, the maximum lacceleration of the dolly during impact is unknown, and cannot be calculated.

Calibration of acceleration meters, impact recorders, and G meters, such as used in aircraft, has been very laborious, and it has been impossible to perform the necessary accurate calibrations without a substantial amount of test equipment and highly skilled technicians. In most instances, the calibration of such instruments cannot be accomplished without returning the instrument to a service point which has the required test equipment.

The principal object of therv present invention is to provide simple and inexpensive testing methods and apparatus which are adaptable Ato a wide range of impact tests, and whose accuracy is such that they may be used for Calibrating meters and recorders. Other objects and advantages of the invention will be clear by reference to the following description and the accompanying drawings. `l

In the drawings:

Fig.'2 is an end elevational .view of the apparatus illustrated in Fig. 1;

Fig. 3 is a fragmentary plan view of the appa-, ratus of Fig. 1; and -1 "1 .1; ,preceding -gurea ..515 Fig. 1 is a side elevational view of apparatus" in accordance with the invention;

Testing apparatus, in accordance with the invention, includes a relatively light but strong platform 5 which is supported from a rigid, elevated frame 'I by means of cables 9 or the like so that the platform 5 is free to swing under the influence of gravity from an elevated release position to a lowered or rest position. Conveniently the rest position is at the lowermost point in the arcuate path of movement of the platform. Means are provided for decelerating the platform 5 at a calculable rate sothat the effect of that determined deceleration may be noted on the test specimen. Deceleration and acceleration are numerically equal and have an identical effect, the only difference being a matter of sign. Hence, for thepurposes of this case the term acceleration will be used to denote either effect.

rI'he platform 5 should be rigid and strong, but comparatively light in weight. The platform 5 comprises a rectangular frame I I of metal which is provided with a pair of bracing members I3 which extend between the rear central portion of the frame to the forward corners of the frame. A stiff plate I5 is rigidly attached to the upper surface of the frame II as illustrated. The objects under test may be attached to the plate I5 by means of screws or bolts.

A bumper block I'I of metal is rigidly attached to the under side of the frame I I at the rear central portion. A tongue member I9, consisting of a heavy bar of metal, extends forwardly from the bumper block I'I and co-acts with a guide means, as will be hereinafter described.

As previously noted, the platform 5 is suspended from a. rigid frame 1, such as the frame of a building, by means of a plurality of spacedapartl cables 9. In the apparatus illustrated, a vertically extending cable 9 connects an eye 2| on each of the four corners of the platform' frame II with the overhead frame 1. Turnbuckles 23 are provided for leveling the platform 5,k and for equally distributing the weight of the platform. Guide cables 9a (Fig. 2), which are inclined `from the vertical, connect the corners of the platform frame I I with the overhead frame 1, as illustrated, to minimize side-sway. The cables, 9 and 9a, at each end of the platform 5 lie substantially in a common vertical plane when the platform is in the undisplaced position, so that the platform is always maintained in a level condition when it is displaced along its fore and aft axis.

The bumper means, for decelerating theplatform 5, comprises a pair of brackets 25 which are rigidly fastened to a stationary base or platform, such as the floor of the building in which the apder impact.

paratus is to be used, by means of heavy bolts 21. A horizontally extending metal bumper bar 29 is supported upon the brackets 25 as a simple beam, by means of clips 3|. The brackets '25 are disposed on a line transverse to the center line of the platform 5 and are located so that the metal bar- 29 contacts the bumper block I1 on the centerline of frame ll when the platform 5 is in the rest position at the bottom of its arc of travel. The metal bar 29 is proportioned so that the tongue member i9, previously described, extends under it to co-act with a roller stop 33. The roller stop 33 comprises a roller 35 supported for rotatable movement upon ,aV central shaft 31 which is journaled into a pair of rigid brackets 39. The height of the roller 35 from its support is such that the tongue member iS slides under the roller 35 when the platform 5 is in the rest position.

An arcuate scale 3B is provided having a radius equallto'the Ylength of the vertical supporting cables 9. The scale 39 is divided into graduations yIH' corresponding tothe chord distance of the displacement of the platform 5. A pointer '43' issupported'upon the bumper block Il for :indicating the displacement along the scale 39.

Noord-45 attached toa standard M serves to Yhold the' platform 5 in 'the displaced position, and this cord 45 engages a reieasable spring latch `connection 49'which is usedfto release the platform Vfor swinging motion.

Means (not shown) may be provided for measuring the' maximum deflection of the bar 2a un- -This means may comprise the usual modeling clayand holder, wherein the displacementcf' the bar makesv an indentation in the clay, theV depth of which maybe subsequently measured.

During the'testing ofa meter or other object,

The platform The. platform 5 is thenreleased,

pact of the bumperb'lo'ck Hand theb'ar 29; the

tongue land roller-35 act to prevent the rear edge Vof the platform 5 lfrom riding 'over the bar '.29. The deflection of the bumper 'bar 29 ismeasured and recorded. The impact reading is noted, if' an instrument is the object being tested,

and this reading isV compared with the value obtained by substituting the determined constants inoneof the following formulae:

.-inl which the-symbols have the meanings:

' Grthe so-called "G factor which equals the ac- `celera`tion of the body under test'V divided bythe acceleration of gravity upon a freely falling body; g=the acceleration of gravity on a freely falling body;

a=the acceleration of the body under test;

"p=Wb/W, Wb=weight ofthe bar 29; W=total.weight of the platform and the objects i secured. to it;

`flexural rigidity of the'bar.

rc=chord distance between the rest position of the pendulum platform and the point of release;

A=maximum central deflection of the bar 29;

Rzlength of the vertically extending supporting cables 9; and

Y=forceconstant orstinness of the steel bar (central force' required per unit deflection as determined by previous dead-weight loading).

In order to facilitate solving the equations it isdesirable to accurately plot the curve of values of p against values of C, for the series of tests involved. The difference between a series of values read on vthe meter under test as compared -with the corresponding calculated values enables the preparation of an arcuate calibration chart.

' The v"development of the above equations is briefly outlined below:

Calculation of acceleration of the platform ini cluding correction-for maseofthev burr emplayed to stop the platform The platform 5,"having a mass M; engages a ksteel bar 29 arranged as a simple beam, ofvlength L andl mass Mb. The mass of theA bar is ynot small enough to neglect and, hence, it is necessary to-develop'expressions for theefective mass of thev bar. The impact" between the platform and the bar duringv a` test' inelastic as some kinetic energy is converted to heat, andV not all elements of the bar travel with'the same velocity. The equation of the curve the deformed bar assumes isgiven by Equation 1.

in which' Pis the central load and EI is the 'The ratio of the deiiection y at a point x distance 'fromeither end ofthe bar-to the ycentral deflection yc (at the point of impact) is seen to be tion 2.

The `lcontributicm to the'1 total momentumj'B, of two'elementsinthe bar oflength dx, and/distance from 'the-ends of the bar 'is in which m: Vis themass per .unit length ofthe bar, v is the velocity 'off'the bar atdi'stance x from either end `of .the bar;v and oc'isfthe velocity at the center'of-uthef'bar `v(which is Athe same-as that of the platform) The-total momentum of thebar is obtained vby integrating the above eX- pression:

This' means that %of `the' mass of. the'bar is effectively located' at the: center in .calculations involving momentum.

Efectiva mass of the bar in Aenergy considerations The kinetic energy of tWO-Ielements'inthe bar of length -dand distance a; from the endsof the Integrating Equation 5 to'obtain the totalkinetic energy of the bar,

(aangaf/2mboeouamae 6) Calculation of acceleration of the platform (maximum value) when stopped by flemure of the steel bar Conserving momentum during the impact of the platform and the steel bar,

where u is the velocityof the platform immediately prior to impact: veis the common velocity of the platform and center of the bar after impact. Immediately after the impact with the bar, the kinetic energy of the system is The right side of Equation 8 is obtained by substituting for ne from Equation 7. If h is the vertical fall of the platform prior to impact with the bar,

(1/2)Mu2=Myh (9) if we neglect the trivial energy loss in air friction. Substituting for u2 inrEquation 8 the value obtained from Equation 9, and equating the kinetic energy after impact (Equation 8) to the elastic energy stored in the bar at itsv maximum deflection, We obtain M2[M-i-(17/,5)Mtlgh/[M+ (%)Mbl2=(1/;)KA2 (10) G=2(h/A) [1-|-(1%5)pl/[1+(5/;)pl3

=2C(h/A) in which p=Mb/M, the ratio of the mass of the bar to the mass of the platform, which is the same as the ratio of their Weights and C is The alternative expression for G is obtained by eliminating Afrom Equation 11, using Equation 10:

in which C has the same value as in Equation 12, i. e., C=[l+(17/35)pl/[1|(5/;)pl3, and W and Wb are respectively the Weights (in pounds) of the platform and the bar.

In using Equation 12, the quantities h and A can be measured in inches (since the conversion factor to feet cancels) in using Equation 13, K should be measured in pounds per foot deflection if h is measured in feet-but, if h is measured in inches, K should be expressed in pounds 'per inch deflection.y VVV (which includes the weight of any object rigidly mounted on the platform) and Wb must be expressed in. pounds weight. f

Substituting for h in Equations 12 and 13, values of the chord distance of displacement, and R, the radius of the arc traversed by the platform, obtained from the equation h=x2/2R, weobtain as a result the useful equations whose terms have been previously defined.

Other procedures of'testing may be employed; in one of these vprocedures the platform is equipped at its frontend with a light but rigid bulwark. The object under test is placed upon the platform, in contact with the bulwark, and the platform is raised to the desired point and released. The platform is stopped with a calculable peak acceleration, and the effects of this impact upon the object or its contents are studied. The same object can be subjected to repeated impacts at a given peak acceleration, or withthe acceleration increased with each impact in accordance with a predetermined plan.

The features of our invention that are believed to be new are expressly set forth in the appended claims. f

We claim: l

l. In apparatus for testing objects at calculable accelerations, a pluralityof vertically extending, spaced-apart cables, a generally horizontally disposed platform supported by said cables for swingable movement between a rest position and position, a fixedly positioned guide member,

means for slidably engaging said platform with said guide member along a line parallel to the line of travel of said platform and adjacent said rest position, said guide member being proportioned to substantially prevent vertical movement of said platform in the region of said rest position, means for displacing said platform from said rest position to a predetermined release position, and means for releasing said platform so that it is acted upon by the force of gravity and swings against said beam, whereby acceleration of objects on said platform mair be calculated.

2. An apparatus for testing objects at calculable accelerations, comprising a rigid, elevated frame, a plurality of generally vertically extending, spaced-apart cables attached to said frame, a generally horizontally disposed platform having a surface for supporting various objects undergoing test, said platform being supported by said cables for swingable movement, between a rest position and a release position which is elevated with respect to said rest position said cables being disposed in sets, each of which comprises a pair of transversely spaced, generally vertically extending cables, and the cables in each set being disposed in a single plane which is normal to the center line of said platform when said platform is in said rest position, fixed bump-er means disposed to interrupt and to stop said platform when said platform moves from said elevated release position toward said rest positiomsaid; bumper means including a simple beam having its midpoint located adjacent the center line of the vpath of movement of said platform and a bumperv on said platform for engaging. the midpoint of said simple beam when said platform swings tov said rest position, and means for displacing saidy platformfrom said rest positionto an elevated position a predetermined distance alongthe center line thereof, means for releasing said platform so that the force of gravity acts upon it and swings it against said simple beam, whereby the peak acceleration, acting upon said platform and objects attached thereto upon impact with said beam, may be determined.

3. In apparatus for testing objects, a generally horizontally disposed platform'having a loadsupporting surface upon which various objects under test are disposed, means` for supporting said `platform for swingable movement in an arc of determined radius between a forwardly disposed rest position and a rearwardly disposed release position elevated with respect to said rest position, a beam flxedly pos-itioned in the path of movement of said platform, means on said platform for engaging said beam, a forwardly extending tongue attached to said platform, and a xed guide positioned to receive said tongue when said platform is inA said forwardly disposed rest position, said tongue and said guide co-acting to prevent vertical movement of said platform in the region of said rest position.

4. In an apparatusfor testing objects, a generally horizontally disposed platform having a `load-supporting surface upon which various objects under test are disposed, means for sup porting said platform for swingable movement in an arc of determined radius between a forwardly /csposedz rest position'= and. av rearwardly disposed release position elevated with respect to said rest position, means for interrupting the swinging movement of said platform' closely. adjacent the rest position including a member of uniform .cross-section supported asa simple beam with its midpoint located adjacent the center line of theV path of movement of said platform, said simpleibeam being located adjacent the rearward end of said platform when said platform is in the rest position, a member on said platform for engaging the midpoint of said simple beam when said platform is in the rest position, a forwardly extending tongue attached to the rearward end of said platform and a fixed guide positioned adjacent said rest position for receiving said tongue when said platform is in said forwardly disposed rest position, and a roller on said guide for engaging said tongue,vsaid guide and tongue coacting to prevent vertical movement of the rearward end of said platform when it is in the region of said rest position.

GEORGE R. SEARS.

ROBERT D. RAE.

JOHANNES A. VAN DEN AKKER.

REFERENCES CITED The following references are of. record in the file of this patent:

UNITED STATES PATENTS Number Name Date 403,676 Keep May 21, 1889 25,430,876 Hodges NOV. 18, 1947 FOREIGN PATENTS Number Country Date 396,057 Great Britain July 26, 1933 

